In general, hydrate is a solid substance in the form of ice including water molecules and gas molecules, and the hydrate is created as water and gas molecules come into contact with each other under predetermined pressure and a predetermined temperature. The hydrate is dissociated into water and gas molecules by changing pressure or a temperature, and the hydrate has high gas concentration because the hydrate has a crystal structure in which gas molecules are disposed in spaces made by water molecules, and as a result, the hydrate attracts attention as a new means for transporting and storing natural gas which is substituted for LNG.
That is, it has been known that because the natural gas hydrate has a phase equilibrium (self-preservation) effect under a predetermined temperature and pressure condition, the natural gas hydrate may accommodate methane gas as much as up to 170 times a volume of the natural gas hydrate, and the phase equilibrium may be maintained when the natural gas hydrate is stored and transported at a temperature of about −20° C. Here, as examples in the related art for transporting the natural gas hydrate, for example, Korean Patent No. 10-0840059 discloses a natural gas transporting means configured such that hydrated natural gas is sealed and stored in container type storage containers, the container type storage containers are stacked on a transport ship in a container stack pattern, a hermetic protective cover is installed at an upper side of the transport ship so that the stacked containers are included, a portion between the protective cover and the container type storage container is filled with inert gas, and a constant temperature is maintained in the protective cover by an inert gas cooling unit installed to be connected to the protective cover. In addition, Korean Patent No. 10-1149498 discloses a natural gas hydrate container which includes a main body which has a natural gas hydrate storage space that is sealed or opened by a door, a cooling unit which is installed in a main body so as to cool the storage space, a discharge port which is provided in the main body so as to discharge natural gas from the storage space and controls a discharge of natural gas, and a drain port which is provided in the main body so as to discharge water from the storage space and controls a discharge of water.
Further, as another example in the related art for transporting the natural gas hydrate, for example, Korean Patent No. 10-1262647 discloses a natural gas hydrate loading apparatus capable of efficiently loading a large amount of NGH because the NGH is loaded directly into a cargo hold of an NGH transport ship by using a loading apparatus installed outside the NGH transport ship, such that it is not necessary to install additional equipment for loading the NGH to the NGH transport ship, and thus ship costs and maintenance costs for the NGH transport ship are reduced, and a shape of the cargo hold of the NGH transport ship is efficiently made. In addition, Korean Patent Application Laid-Open No. 10-2014-0065588 discloses an apparatus and a method for unloading gas hydrate which are capable of unloading marine-transported gas hydrate in a gasified state to the ground by using a floating cargo hold separable from the transport ship, and a dock that confines the cargo hold to the interior of the dock.
Various technologies for loading and transporting the natural gas hydrate have been presented in the related art as described above, but the natural gas hydrate transportation method in the related art has the following problems.
That is, because boil-off gas (BOG) is partially generated even in a case in which phase equilibrium of the natural gas hydrate is maintained, the boil-off gas causes an increase in pressure in a transport container such as a tank container, and the amount of generated boil-off gas may be rapidly increased in a case in which a predetermined temperature and pressure condition cannot be satisfied due to damage or a breakdown of the transport container such as the tank container.
Therefore, to minimize the amount of generated boil-off gas and maintain a phase equilibrium condition in the transport container such as the tank container, a refrigerator is installed and operated in the transport container such as the tank container in the related art.
However, in general, there is a practical limitation in connecting electric power lines individually to the transport container such as the tank container, which loads and transports the natural gas hydrate with several thousands of TEU, and operating the refrigerator, and as a result, there is a problem in that to transport a large number of transport containers such as the tank container to a long distance by using the existing ships such as a container ship, it is necessary to maintain an appropriate temperature and pressure condition for maintaining phase equilibrium and to manage boil-off gas (BOG) which occurs naturally.
In more detail, because the transport container such as the tank container is an adiabatic compression container, it is not greatly necessary to supply a heat quantity by the refrigerator in a case in which a transportation distance is short, but in a case in which a transportation distance becomes long, it is necessary to adjust a temperature condition for phase equilibrium by means of the refrigerator and the like because it may be difficult to maintain a temperature and pressure required for phase equilibrium of the adiabatic compression container over time.
In addition, it is necessary to improve a pressure resistance capacity of the transport container in order to prevent the occurrence of a risk that pressure in the transport container such as the tank container is excessively increased due to the boil-off gas (BOG) and the transport container is damaged, but the improvement of the pressure resistance capacity of the transport container causes a problem of a decrease in the amount of loaded natural gas hydrate caused by a limitation in weight for each unit tank container because of an increase in load of the transport container itself, and a problem of an increase in transport container manufacturing costs.
Further, in a case in which the boil-off gas is discharged to the outside of the transport container such as the tank container in order to prevent an increase in internal pressure, there is a problem of the occurrence of fire caused by lightning, friction, or spark, or a problem of environmental pollution.
Furthermore, in the related art, an operation of connecting the electric power line to the refrigerator and an operation of connecting a pipe for discharging boil-off gas need to be manually and individually performed after stacking the respective natural gas hydrate tank containers during transportation of the natural gas hydrate, and as a result, there is a problem in that the operations of connecting the electric power line and the pipe are very inconvenient and require a large amount of time.
Therefore, to solve the problems of the transportation method using the natural gas hydrate tank container in the related art, it is preferred to provide a natural gas hydrate tank container loading system which has a new configuration, may stack and transport a plurality of transport containers such as the tank container without excessively remodeling the existing ship such as a container ship or incurring costs, may stably supply electric power to the refrigerator for maintaining phase equilibrium of the respective transport containers during transportation with a simple configuration and low costs, and may effectively treat boil-off gas generated in the respective transport containers without discharging the boil-off gas to the outside, but an apparatus or a method, which meets all of the requirements, is not yet provided.
In addition, to solve the problems of a transportation method using the natural gas hydrate tank containers in the related art in that the operation of connecting the electric power line to the refrigerator and the operation of connecting the pipe for discharging boil-off gas need to be manually and individually performed after stacking the containers, which causes an inconvenience, it is preferred to provide an electric power line and pipe connecting structure for a natural gas hydrate tank container loading system which has a new configuration, and may automatically connect the electric power line and automatically connect the pipe by simultaneously stacking the respective containers, but an apparatus or a method, which meets all of the requirements, is not yet provided.